$ABCD$ is a quadrilateral with $\angle ABC = 90^\circ$, $\angle ACD = 90^\circ$, $AC = 30$, $BC = 18$ and $AD = 50$. If $BD = a\sqrt{b}$, what is the value of $a + b$?
BD = a√b = 2√769 a + b = 771
